The present invention concerns an electro-luminescent diode, the luminous surface of which is defined by a thinned portion comprising zones having emitting junctions.
Electro-luminescent diodes are used for numerous display devices, for example, for: table calculators, measuring instruments or watches. In many cases their possibilities of use are limited by their insufficient efficiency.
The light produced by an electro-luminescent diode is, in general, created near (of the order of 1.5 .mu.m) its plane luminous surface so as to reduce the absorption of emitted light. However, due to the high refractive index of the epitaxial material (n = 3.5 for the Ga (AsP)) used to produce such a diode, only the light produced in a cone having a small aperture (half angle at the apex .congruent. 16.degree. = sin.sup.-.sup.1 (1/3.5)) can escape from the crystal, Most of the light is lost by total internal reflection in the crystal.
In the case of transparent crystals (Examples GaP (N), GaP(Zn,O)), some of the light which is normally lost, is recovered by the formation of "mesa" diodes, as that, for example, described in "Monolythic displays in GaP", N. E. Schumaker, Bell Labs. International Electron Devices Meeting 1973.
These diodes have the following disadvantages:
INSUFFICIENT CONCENTRATION OF THE LIGHT, LOSSES OF LIGHT THROUGHOUT THE CRYSTAL, DEEP "MESA" STRUCTURE IN WHICH THE INTERCONNECTIONS AND THE USE OF DOMES ARE DIFFICULT.
There are other types of diodes for which the emission is subjected to strong crystalline absorption. This is the case, in general, with direct type radiative recombination crystals, such as GaAs.sub.1.sub.-x P.sub.x (x &lt; 0.45) and GaAs. The position is even more critical, because all the light which undergoes total internal reflection is absorbed in the crystal. Attempts have already been made to reduce the absorption according, for example, to a technique described in: "Improved Technique for the Preparation of Ga.sub.x Al.sub.1.sub.-x As Electroluminescent Diodes", H. Beneking, Electronics Letters 8, 16, 13th January 1972.
The absorption of light issuing directly is reduced by these techniques, but most of the light remains in the crystal and is absorbed after total internal reflection at the surface.
In order to reduce simultaneously losses of absorption and total reflection on the surface, it has already been proposed to locate, above the junction, a transparent compound material, the surface of which is then worked to form domes. A structure of this type is described, for example, in: "Semiconductive alloy light source having improved optical transmissivity", S. V. Galginaitis, assignor to General Electric Company, U.S. Pat. No. 3,302,051, Jan. 31st 1967.
Although these structures are interesting, they have several disadvantages:
loss of light emitted outside the cone defined by the dome;
very high cost, because it is necessary to grow an epitaxial layer during a very long time, and moreover, the domes have to be shaped.
Finally, electroluminescent diodes are known from U.S. Pat. No. 3,703,670 (Kunz) comprising cylindrical junctions disposed perpendicularly relative to a thinned zone which reduces the absorption and comprising further a reflecting surface, serving also as electrode, to recover a small part of the light emitted in the wrong direction. The improvement in the luminosity of the diode is limited, whereas the emitted light is not homogeneous over the entire thinned zone forming, in fact, the luminous surface.